Several metals and metallic compounds, such as silver, copper and zinc, are well known for their anti-microbial properties. In particular, low concentrations of monovalent silver ions are toxic to micro-organisms. Additionally, silver and silver ions are generally considered to be safe for human consumption at the low concentrations that are effective for anti-microbial applications. Accordingly, the use of metallic silver and silver compounds is particularly advantageous for treating water to obtain water with acceptable levels of microbial organisms for human consumption.
There are several different methods of delivering silver and silver ions into water at low concentrations necessary to observe an anti-microbial effect.
For example, water may be passed through a silver-containing media of activated carbon impregnated with silver salts and metallic silver or an ion exchange medium loaded with monovalent silver ions to increase the concentration of monovalent silver ions in solution to a level which is toxic to micro-organisms. Other silver-containing media, such as high surface area substrates comprising silver or silver compounds have also been used, wherein the method of delivery of solvated silver species is reliant on the slow and continuous dissociation of silver cations into solution at the liquid-solid interface of the substrate.
In all but the purest forms of water, solvated halide ions, particularly chloride ions, are ubiquitous. Thus, it is not surprising that the residence time of monovalent silver ions in solution is typically short and of the order of a few minutes, primarily because of formation of a silver halide precipitate arising from the reaction of monovalent silver ions with solvated halide ions, in particular chloride ions.Ag++Cl−→AgCl(s)  (1)
However, the short residence time of monovalent silver ions in solution is considered long enough for an anti-microbial effect to be measurable.
As most water subjected to water treatment will generally contain sufficiently high concentrations of solvated halide ions for silver halide formation to occur, regular replenishment of the water with low concentrations of monovalent silver ions is required to maintain micro-organism populations at acceptable levels for human consumption, particularly if the water is to be stored and not consumed immediately after treatment with silver.
Monovalent silver ions may be rapidly and continuously introduced into aqueous solution by electrolytic means using a silver-containing electrode. However, this method is reliant on an external electrical power source, and it is unsuitable for remote or developing communities where a reliable external power source may be neither available nor affordable.
The effectiveness of all the above described methods is also compromised by the formation of an inert film or coating of silver halide over the liquid-solid interface of the silver-containing media or the silver-containing electrode, which prevents further release of silver into solution.
Similarly, other metals and metal compounds which display anti-microbial properties, such as copper and zinc, are prone to oxidation and the resultant formation of an inert film or coating of metal oxide over the liquid-solid interface of a substrate containing the metals and/or metal compounds.
The efficacy of other forms of silver, such as trivalent silver ions, as anti-microbial agents has also attracted recent attention. In U.S. Pat. No. 5,223,149 Antelman describes treating utilitarian bodies of water, such as swimming pools, hot tubs, municipal and industrial water supplies with a liquid concentrate of soluble Ag(III) complexes, in particular silver(III) periodate and silver(III) biguanide complexes.
Treatment of water by introduction of mixed valency silver compounds has also been examined, although it appears necessary to combine such compounds with an oxidising agent such as persulfate or ozone to afford the required anti-microbial effect. For example, in U.S. Pat. No. 5,211,855 Antelman describes a method of treating utilitarian bodies of water with tetrasilver tetroxide molecular crystals in the presence of oxidizing agents such as persulfate, whereas in U.S. Pat. No. 6,346,201 Felkner describes a water disinfection method employing ozonated tetrasilver tetroxide and compositions comprising ozonated tetrasilver tetroxide.
It will be appreciated that there is thus a need to provide a means of controllably releasing an antimicrobial agent or other active substance into a fluid without the direct need for electric power.
In the art of garment bleaching, European patent publication EP0339674 discloses an attrition method for releasing bleaching agent from a solid carrier to a second solid material, namely a garment to be bleached. Here, attrition between garment and the solid carrier transfers particles of an oxidising agent contained in a cement matrix to moisture present in the garment against which it is abraded. The particles are not released into a fluid that is free-flowing. Furthermore, the attrition method is difficult to control and does not readily enable a controlled release of bleaching agent.
The present invention seeks to overcome at least in part some of the aforementioned disadvantages.
It is to be understood that, although prior art use and publications are referred to herein, such reference does not constitute an admission that any of these form a part of the common general knowledge in the art, in Australia or any other country.